A type of coupling device wherein light waves are coupled from a first into a second waveguide is described in an article by Miller printed in "The Bell System Technical Journal" Vol. 48, No. 7, 1969 page 2066. In this device, two waveguides are arranged on the same substrate in parallel, closely adjacent relationship to one another over a given distance. The fields of light waves conducted in the first waveguide extend into the region of the second waveguide into which they are extended and forwarded. In this device, it is not possible to control the extent of coupling of the light waves between the two waveguides.